Opto-electronic devices and in particular photovoltaic solar cells rely on the absorption of photons for generating electrical charge carriers. Most photovoltaic solar cells use semiconductor materials in the form of a p-n junction. The p-n junction enables collection of the charge carriers and generation of electrical power.
However, the conversion efficiency of photovoltaic solar cells is limited, at least for some part, by recombination losses due to recombination of charge carriers especially at the surfaces of the semiconductor material. Surface passivation is necessary to passivate the dangling bonds at the surface of a semiconductor material. A thin film is usually applied on the exposed surface for passivating the semiconducting material. Different materials can be used for surface passivation. For example, dielectric materials such as thermally grown silicon dioxide or silicon nitride are commonly used in the PV industry. Semiconducting materials having a wider bandgap than the bulk semiconductor can also be used. For example intrinsic amorphous hydrogenated silicon can be used for the passivation on crystalline silicon. The thickness of the passivating layer is generally comprised between one nanometer and a few tens of nanometers, so as to avoid light absorption by the passivating layer.
The conversion efficiency of photovoltaic (PV) solar cells may also be limited by optical losses due to reflections at the front and/or back surface of the PV cell.
In the present document, the front side of an opto-electronic device is the side which is exposed to the incoming light beam, such as sun light in a solar cell, or an incident light beam to be detected on a photodiode detector. The back side of an opto-electronic device is opposite to the front side.
Anti-reflection coating, or ARC, is commonly used to reduce reflection of incoming light on the front surface (see US 2011/0097840). The ARC may consist in a single thin dielectric layer or in a multilayer stack. The thickness and refractive index of each layer of an ARC are selected so as to create destructive interferences between the light beams reflected on each surface or interface in order to reduce the overall reflected beam intensity. For example, a double layer anti-reflection coating comprising a layer of zinc sulfide (ZnS) and a layer of magnesium fluoride (MgF) can be used. Alternatively, two layers of silicon nitride with varying refractive indices can also be used as anti-reflection coating.
Surface texturing can also be used to reduce light reflection especially on the front surface. Moreover, texturing the surface of a thin solar cell enables multiple passes of the light inside the thin solar cell by total internal reflection. Surface texturing thus also enhances light trapping inside the solar cell. Light trapping increases the optical path length of incoming photons inside the solar cell. Because absorption of near infrared photons in silicon requires optical path lengths larger than the cell thickness particularly for thin devices, surface texturing, by increasing light trapping inside the solar cell, thus increases light absorption and generation of charge carriers.
In the past, various techniques have been employed for surface texturing. These texturing techniques may be classified in wet etching and dry etching processes. In particular, the anisotropic wet-etching of a monocrystalline silicon substrate preferentially etches along some crystallographic orientations. The etching of silicon thus may result in a textured surface made up of randomly distributed pyramids, wherein the tops of the pyramids protrude out of the substrate. Other texturing processes, such as plasma etching, may produce other types of surface texturing, such as inverted-type pyramids, wherein the tops of the pyramids are oriented toward the bulk of the silicon substrate (see P. Roca i Cabarrocas et al., “Method of texturing the surface of a silicon substrate, and textured silicon substrate for a solar cell”, US2012/0146194 and see A. Mavrokefalos et al., “Efficient light trapping in inverted nanopyramid thin crystalline silicon membranes for solar cell applications”, Nano Lett., 2012, 12, 2792-2796) or silicon nanowires or silicon nanocones (see Sangmoo Jeong et al. “All-back-contact ultra-thin silicon nanocone solar cell with 13.7% power conversion efficiency”, Nature Comm., 2013, DOI: 10.1038/ncomms3950).
Moreover, masking techniques may also be employed, in combination with either wet or dry etching, to produce periodic surface texturing. Nanostructuring by nanoimprint with dry or wet etching enables forming photonic structures or periodic inverted pyramids at the surface of a solar cell.
The surface passivation of a textured surface, for example pyramid-shaped surface, is more complex than that of a flat surface, due to the higher developed surface and to the presence of inhomogeneities.
The structure of a solar cell generally relies on a planar junction of homojunction or heterojunction type. Screen printed solar cells and buried contact solar cells provide a metal grid contacts on the front surface and back contacts. In such devices, the p-n junction is formed across the device thickness. In contrast, rear contact or Interdigitated Back Contact (IBC) solar cells place both electrodes on the back surface, thus eliminating shadowing losses on the front side. In IBC structures, the p-n junction is formed near the backside of the solar cell. Other types of junctions have also been proposed such as radial junction silicon nanowires, wherein each silicon nanowire has a radial doping profile thus forming a radial p-n junction.
Solar cell devices having textured surfaces thus generally present higher recombination losses than solar cells with a flat surface. These recombination losses are generally attributed to textured surface inhomogeneities and/or imperfect passivation. It has been shown that ordered arrays of silicon nanowires forming radial p-n junctions increase the path length of incident solar radiation by up to a factor of 73, but that there is a competition between advantages in light-trapping, providing improved absorption and disadvantages in surface recombination (see Erik Garnett and Peidong Yang “Light trapping in silicon nanowire solar cell”, Nano Lett., 2010, 10, 1081-1087 or F. Priolo et al. “Silicon nanostructures for photonics and photovoltaics”, Nature Comm., January 2014, DOI: 10.1038/NNANO.2013.271).
Other surface modifications have been proposed to increase light absorption in a solar cell without increasing recombination generally associated with nanostructures. In particular, the formation of needle-shaped structures on a silicon substrate produces a so-called black-silicon surface which provides high absorption of incoming light (see Jinhun Oh, Hao-Chih Yuan and Howard M. Branz “An 18.2%-efficient black-silicon solar cell achieved through control of carrier recombination in nanostructures”, Nature nanotechnology, vol. 7, 2012, pp. 743-748). Auger recombination can be suppressed with light and shallow doping and simultaneous control of the surface area. However, the passivation of a needle-shaped structured surface is difficult. Thus, surface recombination remains an issue in nanostructured silicon solar cells.
Document US 2013/0291336 A1 discloses forming an array of nanorods of zinc oxide or magnesium zinc oxide on a front side surface of a solar cell, a protective layer being formed on the nanorods, these nanorods having a high aspect ratio and a length of a few micrometers. According to US 2013/0291336 A1, the array of nanorods contributes to a low reflectance in a wide range of sunlight wavelength and a wide range of incident angles, thus enabling an increased absorption of sunlight, and in turn a higher solar cell efficiency as compared to a solar cell with a conventional ARC layer.